1. Samsung Semiconductor R&D Center, Hwasung-si, Gyeonggi-do 17113, South Korea
2. Department of Physics and Astronomy, University of California, Riverside, California 92521, United States
3. Advanced Device Research Lab, Semiconductor R&D Center, Samsung Electronics Co., Ltd, 1 amsungjeonja-ro, Hwaseong-si, Gyeonggi-do 17113, Korea
4. Department of Physics, The Ohio State University, Columbus, Ohio 43210, United States
5. Department of Physics, University of California, Santa Cruz, California 95064, United States
6. Sorbonne Université, CNRS, Institut des Nanosciences de Paris, UMR7588, F-75252 Paris, France
7. Dipartimento di Fisica, Università di Roma La Sapienza, Piazzale Aldo Moro 5, I-00185 Roma, Italy
8. Department of Physics, University of Trento, Via Sommarive 14, 38123 Povo, Italy
9. Graphene Labs, Fondazione Istituto Italiano di Tecnologia, Via Morego, I-16163 Genova, Italy